Abstract
This study presented the investigation on indirect Cr (VI) bioreduction process by anaerobic sludge produced from kitchen waste (ASKW) using an external source of glucose and sulfate to favor the reducing environment. These compounds were added at the beginning of the experiment along with 500 mg·L−1 Cr (VI). The system containing 1 g of glucose and 2 g of sulfate attained a higher reduction, which was 10% higher than that of the control experiment. This study indicated that a neutral environment (pH ~7), along with a high release of polysaccharides (PS), improved the removal efficiency by Cr (VI) bioreduction process. Desulfovibrio and Sulfurospirillum (genus level), which accounted for 3% and 1% of the whole microorganism, respectively, were responsible for the sulfidogenic reaction. Additionally, Thermovirga (genus level) reduced from 14% to 11% and 10%. These microorganisms contributed to dominating the indirect Cr (VI) bioreduction process. SEM and FTIR analysis of the sludges obtaining from the indirect Cr (VI) bioreduction systems indicated that the external glucose could facilitate the formation of looser porous structures and richer functional groups of sludges, thus adsorbing more Cr (III) to reduce its toxicity. Meanwhile, the intensity of the hydroxyl bond, which possesses strong reducibility, was much higher after adding external glucose. Chromate reductase gene (chrR) and sulfite reductase gene (dsrA) contributed to the indirect Cr (VI) bioreduction process. These might be the main mechanisms of the external glucose acting on indirect Cr (VI) bioreduction by ASKW.
Highlights
IntroductionChromium is frequently observed in trivalent (Cr (III)) and hexavalent (Cr (VI)) forms in natural water [1]
Introduction published maps and institutional affilChromium is frequently observed in trivalent (Cr (III)) and hexavalent (Cr (VI)) forms in natural water [1]
It is demonstrated that 65.9% of 500 mg·L−1 Cr (VI) could be removed by anaerobic sludge produced from kitchen waste (ASKW) at 96 h in the CK group, in which 45% of Cr (VI) was reduced by various kinds of reducing substances existed in ASKW, such as humus, citric acid, and fatty acid [27]
Summary
Chromium is frequently observed in trivalent (Cr (III)) and hexavalent (Cr (VI)) forms in natural water [1]. Cr (VI) to Cr (III) is considered a significantly effective approach for removing chromium [2]. Among the various chromium removal methods, biological conversion (especially using bacteria and fungi) has been proven to be attractive and cost-effective because of its high efficiency, low operating cost, short operation time, and eco-friendliness [3,4]. CrO4 2− could be directly reduced to Cr (III) by using organics as electron donors through the cytochromes of microorganisms (Equation (1)). Natural anaerobe metabolites, such as H2 S which are generated by sulfate-reducing bacteria (SRB) in the presence of sulfate, are effective indirect chemical Cr (VI) reductants under iations
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